The initial separation of the numerous stream components contained in an oil or gas well stream is one of the most basic operations in the production of oil and gas. Typically, a hydrocarbon well stream contains numerous components, including natural gas, hydrocarbon liquids, produced water, and particulates (such as sand). It is necessary to separate these four components before the oil and gas may be sold or used in the production operations.
Gravity separation vessels are usually used to separate the well stream components. A typical production facility would include at least two such vessels: a free-water knockout vessel and a production separator. Both vessels have a steel shell with internal weirs and baffles. During production, the well stream would be produced through the free-water knockout vessel to remove a large portion, generally 60%-90%, of the free water from the well stream. The separator then further separates the remaining well stream components of gas, oil, and produced water into the individual components. The oil is discharged from the production separator to another vessel for additional treating for sale. The water from the production separator is discharged and sent to yet another vessel where the small amount of oil that may have remained in the water is removed. This treated water will then be handled as waste. The gas component also exits the production separator and is sent to a gas handling facility where it will be treated for sale or use. Any sand produced will accumulate in the free-water knockout vessel and production separator until these vessels are removed from service and cleaned.
As is seen from this brief description, many pieces of separation equipment are typically used in the production of oil and gas. Each piece is expensive to install, maintain, and operate.
The weight and space requirements of the separation equipment is of particular concern for an offshore platform. When offshore production facilities are mounted on platforms in tens, hundreds, or even thousands of feet of water, space is extremely expensive to provide. Reducing the size and weight of each piece of equipment can reduce the size of the offshore platform. It is on the offshore platform that this present invention will be such a valuable piece of equipment. There is a need for a single, small, relatively light weight piece of equipment which separates relatively large volumes of oil, gas, and water and which replaces the large, heavy, expensive, vessels used in the past.
Centrifugal equipment has been suggested for use in separating the multiple components of an oil or gas stream. In the typical arrangement the well stream fluids are introduced in the separator and rotated against the centrifuge wall. Layers of the individual components are formed with the specific gravities of the component layers decreasing as the distance from the wall increases. After separation is completed, the individual separated layers are then removed. This removal process can be extremely difficult. As stated in U.S. Pat. No. 3,791,575 (cl 1 1n 15-18), the flow control of the discharge fluids from a centrifugal separator presents a significant problem to the operation of a centrifuge. Various level control systems for centrifugal separators have been proposed to control the levels and the continuous separation of the inlet stream. Examples of level control systems include inlet controls as described in U.S. Pat. No. 1,794,452; differential pressure controls as described in U.S. Pat. No. 4,687,572; flow rate controls as described in U.S. Pat. No. 2,941,712; discharge fluid analysis as described in U.S. Pat. No. 4,622,029; water recirculation control as described in U.S. Pat. No. 3,208,201; and an adjustable overflow weir control as described in U.S. Pat. No. 4,175,040.
Depending on the service efficiency required by a given centrifuge separator, the above described centrifuges and their respective fluid level control systems can be effective and adequate. The principal disadvantage with the centrifugal systems disclosed in the past is their inability to obtain essentially complete separation of the well stream components. A partial separation of fluids is often not acceptable.
In an offshore oil and gas operation where produced water will be disposed by placing the produced water back into the body of water where the platform is located, it is desirable that nearly no oil (usually less than 50 parts per million) be contained in the disposed water.
In an onshore operation, such complete separation is also desirable where produced water is disposed through disposal or injection wells. If oil is contained in water that is injected into a disposal well, the oil will eventually plug the formation and will require workover expenses to regain water disposal or injection capabilities.
The present centrifuge and level control system is intended to reliably separate the oil, gas, water, and sand components of a well stream.